In a fusion, or vitreous, type seal, the sealing material must be fused, that is, softened or melted so that it flows and wets the surface to which it is applied. A fusion sealing material may be applied to a single sealing surface, as in glazing or enameling. Alternatively, the material may be employed to join two opposed surfaces. The result is then referred to as an intermediate seal or joint. The present invention is applicable to either type of sealing operation.
Fusion sealing is a well cultivated art. In particular, numerous special sealing glasses have been developed for use in joining glass parts with each other, or with metals, alloys, or ceramics.
In making a fusion-type seal, the material must be heated to a temperature where it becomes soft enough to wet the sealing surface and form an adherent, hermetic bond. For many purposes, it is desirable to maintain the sealing temperature as low as possible. This is particularly true in electrical and electronic articles where thermally sensitive parts or coatings are commonly employed.
Accordingly, considerable attention has been given to lead glasses as low temperature sealing glasses. For example, stable sealing glasses, having softening points in the 430.degree.-500.degree. C. range and coefficients of thermal expansion in the 70-90.times.10.sup.-7 /.degree.C. range, are disclosed in U.S. Pat. No. 2,642,633 (Dalton). Subsequent studies centered on lead zinc borate type glasses that were subject to thermal devitrification or crystallization. These glasses were intensively investigated in the search for cathode ray tube sealing materials.
Glasses having even lower transition temperature (Tg) values are disclosed in U.S. Pat. No. 4,314,031 (Sanford et al.). The transition temperature of a glass is the temperature at which the glass is deemed to be transformed from the solid to the liquid state, as determined by standard scanning calorimetry techniques. It commonly approximates the temperature-viscosity value known as the annealing point of a glass.
The glasses disclosed in the Sanford et al. patent are known as tin-phosphorus oxyfluoride glasses. They comprise, in weight percent on an elemental basis as calculated from the batch, 20-85% Sn, 2-20% P, 3-20% O, 10-36% F, the total Sn+P+O+F being at least 75%. In addition, they may contain up to 25% Pb, up to 12% Zr, up to 10% Fe, up to 3% Ti, up to 1% Ca, up to 3% Ba, up to 2% Zn, up to 12% total of Fe+Ti+Ca+Ba+Zn, up to 3% total of Na+K+Li, up to 4% Al, up to 1% Si and up to 20% total of Cl+Br+I.
The very low transition temperatures of these glasses, frequently in the vicinity of 100.degree. C., suggested their use for low temperature sealing applications. However, use of these glasses has been severely limited by their abnormally high coefficients of thermal expansion (CTEs). Thus, the CTE values for the glasses disclosed in the Sanford et al. patent tend to be in the neighborhood of 200.times.10.sup.-7 /.degree.C. Most sealing applications involve glasses, metals, or ceramics having CTE values not over about 100.times.10.sup.-7 /.degree.C. These include the commonly used soda lime and borosilicate glasses, alumina ceramics and many metals and alloys.
The sealing art has recognized that high CTE values can be lowered by mill additions of materials having lower CTE values. Numerous additives have been proposed, particularly for use with the lead borate and lead zinc borate glasses. Among these are titanates, zircon and quartz. Also, crystalline materials having negative CTEs, such as beta-eucryptite, have been proposed.
In selecting a mill addition for CTE control, various other factors must also be considered. For example, it is important that the material be relatively inert. In particular, a mill addition should not react with, or dissolve in, the glass to which it is added. Not only will the CTE control be lost or modified, but such undesirable effects as bubbling, devitrification, and/or physical property changes may occur. Some additives, especially if used in large amounts, may sharply raise viscosity. This adversely affects flow characteristics during sealing.